Exhaust gas-purifying catalyst

ABSTRACT

An object of the present invention is to provide an exhaust gas-purifying catalyst containing a composite oxide catalyst and a refractory support and being less prone to cause a decrease in its activity even when used at high temperatures in an atmosphere with high oxygen concentration. An exhaust gas-purifying catalyst includes a composite oxide catalyst containing rare-earth element, alkaline-earth element and precious metal, a part of the rare-earth element and a part of the alkaline-earth element forming composite oxide, and the composite oxide and a part of the precious metal forming solid solution, and a refractory support supporting the composite oxide catalyst and including at least one of a first composite oxide represented by a general formula AB 2 O 4 , a second composite oxide having perovskite structure represented by a general formula LMO 3 , and a third composite oxide having pyrochlore structure represented by a general formula X 2 Y 2 O 7 .

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP2006/324460, filed Dec. 7, 2006, which was published under PCTArticle 21(2) in Japanese.

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2005-370083, filed Dec. 22, 2005,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas-purifying catalyst.

2. Description of the Related Art

As an exhaust gas-purifying catalyst that treats exhaust gas of anautomobile, a three-way catalyst with precious metal such as platinumsupported by an inorganic oxide such as ceria or alumina has been widelyused. In the three-way catalyst, the precious metal plays the role inpromoting the reduction of nitrogen oxides and the oxidations of carbonmonoxide and hydrocarbons. Further, the inorganic oxide plays the rolesin increasing the specific surface area of the precious metal andsuppressing the sintering of the precious metal by dissipating heatgenerated by the reactions. In particular, ceria has an oxygen storagecapacity and is capable of optimizing the oxidation and reductionreactions.

In recent years, occasions when the automotive vehicle such asautomobile is driven at high-speed increase as the performance of anengine increases. Additionally, in order to prevent pollution of theair, the regulations on the exhaust gas are made more stringent. Againstthese backdrops, temperature of the exhaust gas emitted by theautomotive vehicle is on the trend of rising.

Further, the automotive vehicle is required to decrease the carbondioxide emission in order to suppress the global warming. For thesereasons, occasions when the supply of fuel to the engine is cut off inthe state that the exhaust gas-purifying catalyst is heated to hightemperatures are increasing.

That is, the exhaust gas-purifying catalyst is used at temperatureshigher than in the past, and occasions when exposed to an atmosphereexcessive in oxygen at high temperatures are increasing. For that, inorder to provide the exhaust gas-purifying catalyst that delivers asufficient performance even when used under such a condition, researchand development are actively carried out.

For example, JP-A 5-168926 (KOKAI), JP-A 6-75675 (KOUHYO), and JP-A2000-169148 (KOKAI) describe improving the heat stability of ceria tosuppress the reduction in its oxygen storage capacity and the like.Specifically, JP-A 5-168926 (KOKAI) describes an exhaust gas-purifyingcatalyst containing an element of platinum group, activated alumina,cerium oxide, barium compound and zirconium compound. JP-A 6-75675(KOUHYO) describes an exhaust gas-purifying catalyst in which acatalyst-supporting layer contains cerium oxide, zirconium oxide andcatalytic metal, and at least parts of cerium oxide and zirconium oxideare present as a composite oxide or a solid solution. JP-A 2000-169148(KOKAI) describes a cerium-based composite oxide represented as thegeneral formula: Ce_(1-(a+b))Zr_(a)Y_(b)O_(2-b/2).

Further, JP-A 10-358 (KOKAI) and JP-A 2001-129399 (KOKAI) describemaking platinum present as platinum composite oxide to suppress thesintering of platinum. Specifically, JP-A 10-358 (KOKAI) describes anexhaust gas-purifying catalyst using a high heat-resistant compositeoxide that contains platinum and one or more element selected fromalkaline-earth metal elements and group IIIA elements. JP-A 2001-129399(KOKAI) describes an exhaust gas-purifying catalyst that includes aplatinum composite oxide layer containing platinum and alkaline-earthmetal element on an inorganic oxide support, in which a layer of oxideof metal X, which is at least one element selected from Mg, Ca, Sr, Ba,La and Ce, is interposed therebetween.

However, even if the heat-stability of ceria were improved, thesintering of platinum would occur when the exhaust gas-purifyingcatalysts are exposed to an atmosphere excessive in oxygen at hightemperatures, for example at temperatures from 1,000° C. to 1,200° C.,and a sufficient activity would not be achieved. Also, in order toproduce platinum composite oxide with a high heat-stability, firing athigh temperature is necessary. For this reason, a large majority ofexhaust gas-purifying catalysts using platinum composite oxide are smallin specific surface area and insufficient in activity.

To solve this challenge, the present inventors have proposed a compositeoxide catalyst containing rare-earth element, alkaline-earth element anda precious metal, in which a part of the rare-earth element and a partof the alkaline-earth element form a composite oxide, and this compositeoxide and a part of the precious metal form a solid solution. Thecomposite oxide catalyst exhibits an excellent activity even in the casewhere used at high temperatures in an atmosphere whose oxygenconcentration is high. However, in some cases, the activity of thecomposite oxide catalyst may be lowered when used with a refractorysupport made of alumina.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an exhaustgas-purifying catalyst that contains a composite oxide catalyst and arefractory support and is less prone to cause a decrease in its activityeven when used at high temperatures in an atmosphere whose oxygenconcentration is high.

According to a first aspect of the present invention, there is providedan exhaust gas-purifying catalyst comprising a composite oxide catalystcontaining a rare-earth element, an alkaline-earth element and aprecious metal, a part of the rare-earth element and a part of thealkaline-earth element forming a composite oxide, and the compositeoxide and a part of the precious metal forming a solid solution, and arefractory support supporting the composite oxide catalyst and includingat least one composite oxide selected from the group consisting of afirst composite oxide represented by a general formula AB₂O₄, a secondcomposite oxide having a perovskite structure represented by a generalformula LMO₃, and a third composite oxide having a pyrochlore structurerepresented by a general formula X₂Y₂O₇, the element A beingalkaline-earth element and/or transition metal element, the element Bbeing at least one element selected from the group consisting ofaluminum, magnesium and transition metal elements and differing from theelement A, the element L being rare-earth element and/or alkaline-earthelement, the element M being aluminum and/or transition metal element,the element X being rare-earth element, and the element Y beingzirconium and/or titanium.

According to a second aspect of the present invention, there is providedan exhaust gas-purifying catalyst comprising a composite oxide catalystcontaining a rare-earth element, an alkaline-earth element, zirconiumand a precious metal, a part of the rare-earth element and a part ofzirconium forming a composite oxide with at least a part of thealkaline-earth element, and the composite oxide and a part of theprecious metal forming a solid solution, and a refractory supportsupporting the composite oxide catalyst and including at least onecomposite oxide selected from the group consisting of a first compositeoxide represented by a general formula AB₂O₄, a second composite oxidehaving a perovskite structure represented by a general formula LMO₃, anda third composite oxide having a pyrochlore structure represented by ageneral formula X₂Y₂O₇, the element A being alkaline-earth elementand/or transition metal element, the element B being at least oneelement selected from the group consisting of aluminum, magnesium andtransition metal elements and differing from the element A, the elementL being rare-earth element and/or alkaline-earth element, the element Mbeing aluminum and/or transition metal element, the element X beingrare-earth element, and the element Y being zirconium and/or titanium.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a view schematically showing an exhaust gas-purifying catalystaccording to a first embodiment of the present invention;

FIG. 2 is a conceptual view schematically showing a state change thatthe exhaust gas-purifying catalyst shown in FIG. 1 exhibits under hightemperature conditions;

FIG. 3 is a view schematically showing an exhaust gas-purifying catalystaccording to a second embodiment of the present invention; and

FIG. 4 is a graph showing X-ray diffraction spectra of exhaustgas-purifying catalysts obtained after an endurance test.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below.

FIG. 1 is a view schematically showing an exhaust gas-purifying catalystaccording to a first embodiment of the present invention. The exhaustgas-purifying catalyst is a pellet catalyst formed by agglomerating amixture of a particulate composite oxide catalyst 1 and a particulaterefractory support 2, and a part thereof is shown in FIG. 1.

The composite oxide catalyst 1 includes a support 11, a composite oxide12 partially covering the surface thereof, and a precious metal 13 asupported by the support 11.

The support 11 contains a rare-earth oxide as a main component, whilethe composite oxide 12 contains a composite oxide of a rare-earthelement and an alkaline-earth element as a main component. Therare-earth element forming the composite oxide 12 is the same as therare-earth element forming the support 11. The composite oxide 12further contains the same precious metal as the precious metal 13 a toform a solid solution.

Here, as an example, it is assumed that the support 11 is made of ceria(CeO₂), the composite oxide 12 is made of a composite oxide representedby a chemical formula: BaCeO₃, and the precious metal contained in thecomposite oxide and the precious metal 13 a are platinum (Pt). That is,it is assumed that cerium is used as the rare-earth element, barium isused as the alkaline-earth element, and platinum is used as the preciousmetal. Note that the solid solution of the above composite oxide andplatinum can be represented by the chemical formula: Ba(Ce,Pt)O₃ and/or(Ba,Pt)CeO₃.

The refractory support 2 supports the composite oxide catalyst 1. Therefractory support 2 contains at least one of the following first tothird composite oxides, and is typically made of at least one of thefirst to third composite oxides.

The first composite oxide is a composite oxide represented by a generalformula: AB₂O₄. The element A is alkaline-earth element and/ortransition metal element. The element B is at least one element selectedfrom the group consisting of aluminum, magnesium and transition metalelements, and differs from the element A. Although the first compositeoxide typically has a spinel structure, it may have other crystalstructures such as chrysoberyl structure.

Examples of transition metal element usable for the elements A and Binclude metal elements having an atomic number from 21 to 30. As theelement A, magnesium, strontium or barium can be used, for example.Typically, the element B is aluminum or the major part thereof isaluminum. It is possible that aluminum is used as a part of the elementB, and a transition metal element such as iron and/or cobalt is used asanother part of the element B.

The second composite oxide is a composite oxide having a perovskitestructure represented by a general formula: LMO₃. In the generalformula, L is rare-earth element and/or alkaline-earth element, and M isaluminum and/or transition metal element.

As the element L, lanthanum or neodymium can be used, for example.Examples of transition metal element usable for the element M includemetal element having an atomic number from 21 to 30. Typically, theelement M is aluminum or the major part thereof is aluminum. It ispossible that aluminum is used as a part of the element M, and atransition metal element such as iron and/or cobalt is used as anotherpart of the element M.

The third composite oxide is a composite oxide having a pyrochlorestructure represented by a general formula: X₂Y₂O₇. In the generalformula, X is rare-earth element, and Y is zirconium and/or titanium. Asthe element X, neodymium or lanthanum can be used, for example.Typically, the element Y is zirconium or a major part thereof iszirconium.

The exhaust gas-purifying catalyst exhibits a reversible change in statewhen a composition of an atmosphere is changed under high temperatureconditions. This will be described with reference to FIG. 2.

FIG. 2 is a conceptual view schematically showing a state change thatthe exhaust gas-purifying catalyst shown in FIG. 1 exhibits under hightemperature conditions. In FIG. 2, the state indicated as “Lean” showsthe state that the exhaust gas-purifying catalyst exhibits when exposedto an atmosphere with a high oxygen concentration under high temperatureconditions, for example, when the fuel supply to an engine is cut off.The state indicated as “Rich” shows the state that the exhaustgas-purifying catalyst exhibits when exposed to an atmosphere with a lowoxygen concentration under high temperature conditions, for example,when an abundance of fuel is continuously supplied to an engine, forexample.

The state indicated as “Lean” in FIG. 2 corresponds to the statedescribed with reference to FIG. 1. Here, at least a part of theprecious metal 13 a may be oxidized; in other words, its oxidationnumber may be increased.

In this state, the precious metal 13 a contributes to the activity ofthe exhaust gas-purifying catalyst, while platinum in the compositeoxide 12 hardly contributes to the activity. However, during the periodover which the exhaust gas-purifying catalyst is in the state indicatedas “Lean”, a concentration of offensive components such as nitrogenoxides, carbon monoxide, hydrocarbons, and the like in the exhaust gas,that is, an offensive component concentration in an atmosphere isrelatively low. Thus, the exhaust gas-purifying catalyst delivers asufficient performance.

When the oxygen concentration in the atmosphere is lowered under hightemperature conditions, the exhaust gas-purifying catalyst causes achange from the state indicated as “Lean” to the state indicated as“Rich”. Specifically, platinum is precipitated out of the compositeoxide 12, and the precipitated platinum forms the precious metals 13 bon the surfaces of the composite oxide 12.

During the period over which the exhaust gas-purifying catalyst is inthe state indicated as “Rich”, the offensive component concentration inthe exhaust gas is relatively high. That is, during the periodcorresponding to the state indicated as “Rich”, the exhaustgas-purifying catalyst is required to be higher in activity as comparedto the period corresponding to the state indicated as “Lean”.

The precious metal 13 b is much smaller in size than the precious metal13 a. For example, the size of the precious metal 13 a is severalnanometers, while the size of the precious metal 13 b is equal to orless than about 1 nm. Thus, the exhaust gas-purifying catalyst in thestate indicated as “Rich” is higher in activity than the exhaustgas-purifying catalyst in the state indicated as “Lean”. Therefore, theexhaust gas-purifying catalyst delivers a sufficient performance evenwhen the offensive component concentration in the exhaust gas is high.

The exhaust gas-purifying catalyst in the state indicated as “Rich”causes a change to the state indicated as “Lean” when the oxygenconcentration in the atmosphere increases under high temperatureconditions. That is, platinum forming the precious metal 13 b and thecomposite oxides form the solid solution. Note that platinum and ceriahardly form a solid solution.

As described above, the exhaust gas-purifying catalyst causes areversible change in state. In addition, the exhaust gas-purifyingcatalyst forms the ultrafine precious metals 13 b on the surfaces of thecomposite oxide 12 every time it causes the change from the stateindicated as “Lean” to the state indicated as “Rich”. Therefore, thisstate is recovered by the change from the state indicated as “Rich” tothe state indicated as “Lean” and its reverse change. Since anautomotive vehicle changes the oxygen concentration in the exhaust gasat relatively close intervals, the exhaust gas-purifying catalyst alwaysexhibits a high activity to deriver a sufficient performance whenexposed to a low oxygen concentration atmosphere at high temperatures.

Also, in the exhaust gas-purifying catalyst, the precious metal 13 acontributes to the activity of the exhaust gas-purifying catalystregardless of the composition of the atmosphere and temperature.Therefore, the exhaust gas-purifying catalyst delivers a sufficientperformance not only when exposed to a high oxygen concentrationatmosphere at high temperatures, but also when used for the first timeor used under low temperature conditions.

Further, when the oxygen concentration in the atmosphere is increasedunder high temperature conditions, the exhaust gas-purifying catalystmakes the precious metal 13 b and the composite oxide form the solidsolution as described above. Thus, the exhaust gas-purifying catalyst islow in the evaporation loss of platinum in the high oxygen concentrationatmosphere.

When alumina is used in the refractory support 2, a reaction betweenalumina and the composite oxide 12 occurs under high temperatureconditions of 1000° C. or higher. For example, a reaction betweenalumina and the composite oxide 12 produces BaAl₂O₄ or produces BaAl₂O₄and BaCO₃. When such a decomposition reaction of the composite oxide 12occurs, the activity of the exhaust gas-purifying catalyst is loweredsignificantly.

In contrast, the above first to third composite oxides do not react withthe composite oxide catalyst 1 even under high temperature conditions of1,000° C. or higher. Therefore, the exhaust gas-purifying catalyst isless prone to cause a decrease in its activity due to decompositionreaction of the composite oxide 12. That is, the exhaust gas-purifyingcatalyst according to the present embodiment is less prone to cause adecrease in its activity even when used at high temperatures in anatmosphere whose oxygen concentration is high.

Without willing to be bound by any theory, the reason why alumina reactswith the composite oxide catalyst 1 and the first to third compositeoxide do not react with the composite oxide catalyst 1 is thought to beas follows. That is, this is because the first to third composite oxidesare highly stable materials in the presence of alkaline-earth element,rare-earth element and transition metal element.

The exhaust gas-purifying catalyst can be manufactured, for example, bythe following method.

First, a powdery support 11 containing a rare-earth oxide as a maincomponent is prepared, and is made into slurry. Here, as the dispersionmedium, water is used, for example. Then, a solution of precious metalsalt is added to the slurry, and the resultant mixture is filtrated.Thereafter, drying and firing of the filter cake are carried outsequentially. In this way, the precious metal is supported by thesupport 11.

Next, the support 11 supporting the precious metal is added to asolution of alkaline-earth salt. Then, the slurry is heated so as tosufficiently remove liquid. Thus, the alkaline-earth element issupported by the support 11.

The method of making the support 11 support the alkaline-earth elementis not limited. For example, a method that the support 11 supporting theprecious metal is impregnated with the solution of the alkaline-earthsalt, a method utilizing coprecipitation, a method using an alkoxide ofalkaline-earth metal, and the like may be used.

Then, the support 11 supporting the precious metal and thealkaline-earth element is fired in an oxidizing atmosphere. Thus, thecomposite oxide of the rare-earth element and the alkaline-earth elementtogether with the solid solution of the composite oxide and the preciousmetal are produced so as to obtain the composite oxide catalyst 1.

Note that the firing temperature is set, for example, within the rangefrom about 700° C. to about 1,100° C. When the firing temperature islow, production of the composite oxide is difficult. When the firingtemperature is high, the specific surface area of the support 11 isdecreased, and therefore, it becomes difficult to satisfactorilydistribute the precious metal 13 a over the support 11. As a result, ahigh activity may not be obtained.

Then, the composite oxide catalyst 1 and a powdery refractory support 2are mixed together. Further, the mixture is subjected tocompression-molding, and if necessary, the molded product is crushed.The exhaust gas-purifying catalyst in the form of pellets is obtained bythe above method.

Next, the second embodiment of the present invention will be described.

FIG. 3 is a view schematically showing an exhaust gas-purifying catalystaccording to a second embodiment of the present invention. The exhaustgas-purifying catalyst is a pellet catalyst formed by agglomerating amixture of a particulate composite oxide catalyst 1 and a particulaterefractory support 2, and a part thereof is shown in FIG. 3.

The exhaust gas-purifying catalyst according to the second embodiment isthe same as the exhaust gas-purifying catalyst according to the firstembodiment except that the composition of the composite oxide catalyst 1differs. Therefore, the description concerning the refractory support 2will be omitted.

In the exhaust gas-purifying catalyst according to the secondembodiment, the composite oxide catalyst 1 contains a support 11,composite oxides 12 a to 12 c partially covering the surface thereof,and a precious metal 13 a supported by the support 11.

The support 11 contains a rare-earth oxide as a main component. Thesupport 11 can further contain zirconia (ZrO₂), for example. The support11 may contain a composite oxide of rare-earth element and zirconium asa main component.

The composite oxide 12 a contains a composite oxide of rare-earthelement and alkaline-earth element as a main component. The compositeoxide 12 b contains a composite oxide of zirconium and alkaline-earthelement as a main component. The composite oxide 12 c contains acomposite oxide of rare-earth element, zirconium and alkaline-earthelement as a main component.

The rare-earth elements contained in the composite oxides 12 a to 12 care the same as the rare earth element contained in the support 11, andthe composite oxides 12 a to 12 c contain the same alkaline-earthelement. The composite oxides 12 a to 12 c contain the same preciousmetal as the precious metal 13 a to form solid solutions.

Here, as an example, it is assumed that the support 11 contains ceria asa main component, the composite oxide 12 a is made of the compositeoxide represented by the chemical formula: BaCeO₃, the composite oxide12 b is made of the composite oxide represented by the chemical formula:BaZrO₃, and the composite oxide 12 c is made of the composite oxiderepresented by the chemical formula: Ba(Zr, Ce)O₃. It is also assumedthat the precious metals contained in the composite oxides 12 a to 12 cand the precious metal 13 a are platinum. That is, it is assumed thatcerium is used as the rare-earth element, barium is used as thealkaline-earth element, and platinum is used as the precious metal. Notethat the solid solution of the composite oxide 12 a and platinum can berepresented by the chemical formula: Ba(Ce,Pt)O₃, the solid solution ofthe composite oxide 12 b and platinum can be represented by the chemicalformula: Ba(Zr,Pt)O₃, and the solid solution of the composite oxide 12 cand platinum can be represented by the chemical formula: Ba (Zr, Ce,Pt)O₃.

The exhaust gas-purifying catalyst exhibits a reversible change in statesimilar to the exhaust gas-purifying catalyst according to the firstembodiment when a composition of an atmosphere is changed under hightemperature conditions. In addition, the composite oxide catalyst 1contained in the exhaust gas-purifying catalyst and the above describedfirst to third composite oxides do not react together even under hightemperature conditions of 1,000° C. or higher. That is, similar to theexhaust gas-purifying catalyst according to the first embodiment, theexhaust gas-purifying catalyst according to the present embodiment isless prone to cause a decrease in its activity even when used at hightemperatures in an atmosphere whose oxygen concentration is high.

The exhaust gas-purifying catalyst can be manufactured, for example, bythe following method.

First, a powdery support 11 containing a composite oxide of rare-earthelement and zirconia as a main component is prepared, and is made intoslurry. Here, as the dispersion medium, water is used, for example.Then, a solution of precious metal salt is added to the slurry, and theresultant mixture is filtrated. Thereafter, drying and firing of thefilter cake are carried out sequentially. In this way, the preciousmetal is supported by the support 11.

Next, the support 11 supporting the precious metal is added to asolution of alkaline-earth salt. Then, the slurry is heated so as tosufficiently remove liquid. Thus, the alkaline-earth element issupported by the support 11.

The method of making the support 11 support the alkaline-earth elementis not limited. For example, a method that the support 11 supporting theprecious metal is impregnated with the solution of the alkaline-earthsalt, a method utilizing coprecipitation, a method using an alkoxide ofalkaline-earth metal, and the like may be used.

Then, the support 11 supporting the precious metal and thealkaline-earth element is fired in an oxidizing atmosphere. Thus, thecomposite oxides 12 a to 12 c and the solid solutions of the compositeoxides 12 a to 12 c and the precious metal are produced so as to obtainthe composite oxide catalyst 1.

Note that the firing temperature is set, for example, within a rangefrom about 700° to about 1,000° C. When the firing temperature is low,productions of the composite oxides 12 a to 12 c and the solid solutionsof the composite oxides 12 a to 12 c and the precious metal aredifficult. When the firing temperature is high, the specific surfacearea of the support 11 decreases, and therefore, it becomes difficult tosatisfactorily distribute the precious metal 13 a over the support 11.As a result, a high activity may not be obtained.

Then, the composite oxide catalyst 1 and a powdery refractory support 2are mixed together. Further, the mixture is subjected tocompression-molding, and if necessary, the molded product is crushed.The exhaust gas-purifying catalyst in the form of pellets is obtained bythe above method.

In the exhaust gas-purifying catalyst according to the first and secondembodiment, the content of the composite oxide catalyst 1 is set, forexample, within the range from 1% to 99% by weight.

The ratio of the precious metal forming the solid solution with respectthe whole precious metal supported by the composite oxide catalyst 1,which is referred to as a solid solution-forming ratio hereinafter, isset, for example, within a range from 10% to 80%. When the solidsolution-forming ratio is small, the effect of suppressing the decreasein activity due to the sintering of precious metal may be insufficient.When the solid solution-forming ratio is large, the initial activity maybe insufficient.

Although in the first and second embodiments, the case where cerium isused as the rare-earth element is described as an example, anotherelement may be used as the rare-earth element. For example, lanthanum,praseodymium, neodymium and the like may be used. Alternatively, pluralrare-earth elements may be used.

Although in the first and second embodiments, barium is used as thealkaline-earth element of the composite oxide catalyst 1, an elementother than barium may be used as the alkaline-earth element. Forexample, strontium, calcium, magnesium and the like may be used.Alternatively, plural alkaline-earth elements may be used.

In the exhaust gas-purifying catalyst according to the first embodiment,the atomic ratio of alkaline-earth element with respect to therare-earth element in the composite oxide catalyst 1 is set, forexample, within a range from 1 atomic % to 80 atomic %, and typicallywithin a range from 10 atomic % to 50 atomic %. In the case where theatomic ratio of alkaline-earth element with respect to the rare-earthelement is small, the volume ratio of the composite oxide 12 withrespect to the support 11 is small. Thus, the recovery in performance ofthe composite oxide catalyst 1 caused by the composition fluctuation ofthe atmosphere may be insufficient. In the case where the atomic ratioof alkaline-earth element with respect to the rare-earth element isexcessively large, oxidation, i.e., increase in oxidation number maybecome difficult to occur when the oxygen concentration in atmosphere isincreased at high temperatures. Thus, it may become difficult to producethe solid solution of the precious metal and the composite oxide whenthe concentration of oxygen in atmosphere is increased at hightemperatures, and as a result, the sintering of precious metal may beprone to occur.

In the exhaust gas-purifying catalyst according to the secondembodiment, the atomic ratio of alkaline-earth element with respect tothe sum of rare-earth element and zirconium in the composite oxidecatalyst 1 is set, for example, equal to or higher than 0.1 atomic % andlower than 10 atomic %, and typically within a range from 0.3 atomic %to 5 atomic % in the case where used under high temperature conditionsof equal to or higher than 700° C. and lower than 1000° C. The ratio isset, for example, 10 atomic % or higher, and typically 20 atomic % orhigher in the case where used under high temperature conditions of1,000° C. or higher. Also in this case, the above ratio is set, forexample, 100 atomic % or lower, and typically 80 atomic % or lower. Inthe case where the atomic ratio is small, the volume ratio of thecomposite oxide 12 with respect to the support 11 is small. Thus, therecovery in performance of the composite oxide catalyst 1 caused by thecomposition fluctuation of the atmosphere may be insufficient. In thecase where the atomic ratio is excessively large, the ratio of preciousmetal 13 a with respect to whole precious metal supported by thecomposite oxide catalyst 1 is small. Thus, a sufficient catalyticactivity may not be achieved under high-temperature and high-oxygenconcentration conditions. In addition, when the atomic ratio is raisedexcessively, the heat resistance performance of the support 11 may bedecreased under high-temperature conditions, and as a result, thesintering of precious metal may be prone to occur.

Although platinum is used as the precious metal in the first and secondembodiments, an element other than platinum may be used as the preciousmetal. For example, platinum group elements such as palladium andrhodium may be used. Alternatively, plural precious metals may be used.

The precious metal content of the composite oxide catalyst 1 is set, forexample, within a range from 0.01% to 10% by weight, and typicallywithin a range from 0.1% to 5% by weight. When the precious metalcontent is small, a sufficient catalytic activity may not be achieved.When the precious metal content is large, the sintering of preciousmetal may be prone to occur.

Although the case where the exhaust gas-purifying catalyst is a pelletcatalyst is described as an example, the exhaust gas-purifying catalystmay take various forms. For example, the exhaust gas-purifying catalystmay be a monolith catalyst.

Examples of the present invention will be described below.

EXAMPLE 1 Preparation of Composite Oxide Catalyst A

Cerium nitrate [Ce(NO₃)₃] and zirconium oxynitrate [ZrO(NO₃)₂] wereweighed such that the atomic ratio of cerium to zirconium was 1:1 andwere added to 500 mL of deionized water. After stirring sufficiently, anaqueous solution containing 10% by weight of ammonium hydroxide wasdropped into the aqueous solution at room temperature to causecoprecipitation. The aqueous solution containing the coprecipitate wasstirred for 60 minutes and then filtrated.

The filter cake was sufficiently washed with deionized water and driedat 110° C. The dried material was subjected to a calcination at 500° C.for 3 hours in the atmosphere. The calcined material was crushed byusing a mortar and fired at 800° C. for 5 hours in the atmosphere.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the powder thus obtained. As a result,it was proved that the powder was made of an oxide represented by achemical formula: (Ce,Zr)O₂. Note that the specific surface area of thepowder was 90 m²/g.

Next, 50 g of the oxide powder was weighed and added into 500 mL ofdeionized water. After the oxide powder was well dispersed in thedeionized water by 10 minutes of ultrasonic agitation, a solution ofdinitrodiamine platinum nitrate was added to the slurry. Theconcentration and amount of the dinitrodiamine platinum nitrate solutionwere adjusted such that the platinum content in the composite oxidecatalyst to be prepared would be 1% by weight.

After that, the slurry was filtrated under suction. The filtrate wassubjected to inductively coupled plasma (ICP) spectrometry. As a result,it was revealed that the filter cake contained almost the entireplatinum in the slurry.

Next, the filter cake was dried at 110° C. for 12 hours. Then, it wascalcined at 500° C. in the atmosphere. Thus, platinum was supported bythe oxide.

Subsequently, barium acetate was dissolved into 100 mL of deionizedwater. Then, 50 g of the oxide supporting platinum was weighed and addedinto the barium acetate solution. Note that the concentration of thebarium acetate solution was adjusted such that the atomic ratio ofbarium with respect to the sum of cerium and zirconium in the compositeoxide catalyst to be prepared would be 20.0 atomic %.

Then, the slurry was heated so as to remove the excess water. Next, itwas fired at 1,000° C. for 3 hours in the atmosphere. Thus, a compositeoxide containing barium and a solid solution of the composite oxide andplatinum were produced. Hereinafter, the powder thus obtained isreferred to as a composite oxide catalyst A.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the composite oxide catalyst A. As aresult, it was proved that the composite oxide catalyst A contained acomposite oxide represented by the chemical formula: BaZrO₃ and acomposite oxide represented by the chemical formula: Ba(Ce,Zr)O₃.

A part of the composite oxide catalyst A was taken and immersed for 12hours in a 10% aqueous hydrogen fluoride held at room temperature. Notethat this condition allowed only the barium-containing composite oxideof the above powder to be dissolved. Subsequently, the solution wasfiltrated, and the filtrate was subjected to ICP spectrometry. As aresult, the platinum content of the filtrate revealed that 45% ofplatinum formed the solid solution, in other words, the solidsolution-forming ratio was 45%.

<Preparation of Refractory Support A>

Magnesium acetate [Mg(CH₃COO)₂.4H₂O] and aluminum nitrate[Al(NO₃)₃.9H₂O] were weighed such that the atomic ratio of magnesium toaluminum was 1:2 and were added to deionized water. Here, 107.2 g ofmagnesium acetate and 375.1 g of aluminum nitrate were added to 2,000 mLof deionized water.

After stirring sufficiently, an aqueous solution containing ammoniumhydroxide was dropped into the aqueous solution at room temperature tocause coprecipitation. The aqueous solution containing the coprecipitatewas stirred sufficiently and then filtrated. Note that the aqueoussolution containing ammonium hydroxide was prepared by dissolving 70 gof ammonia into 1,000 mL of deionized water.

The filter cake was sufficiently washed with deionized water and driedat 110° C. The dried material was subjected to a calcination at 600° C.for 3 hours in the atmosphere. The calcined material was crushed byusing a mortar and fired at 1,000° C. for 5 hours in the atmosphere.Hereinafter, the powder thus obtained is referred to as a refractorysupport A.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the refractory support A. As a result,it was proved that the refractory support A was a spinel represented bya chemical formula: MgAl₂O₄. Note that the specific surface area of therefractory support A was 38 m²/g.

<Preparation of Exhaust Gas-Purifying Catalyst AA>

10 g of the composite oxide catalyst A and 10 g of the refractorysupport A were evenly mixed by using a mortar. Next, the mixture wascompression-molded. Further, the molded product was crushed so as toobtain an exhaust gas-purifying catalyst in the form of pellets with aparticle diameter of about 0.5 mm to about 1.0 mm. Hereinafter, theexhaust gas-purifying catalyst is referred to as an exhaustgas-purifying catalyst AA.

EXAMPLE 2 Preparation of Composite Oxide Catalyst B

Barium acetate and calcium acetate were dissolved into 100 mL ofdeionized water. A composite oxide catalyst powder was prepared by thesame method as described for the composite oxide catalyst A except thatthis aqueous solution was used instead of the aqueous solution of bariumacetate. Note that concentrations of barium and calcium in the aqueoussolution were adjusted such that each of the atomic ratios of barium andcalcium with respect to the sum of cerium and zirconium was 10.0 atomic%. Hereinafter, the powder thus obtained is referred to as a compositeoxide catalyst B.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the composite oxide catalyst B. As aresult, it was proved that the composite oxide catalyst B contained acomposite oxide represented by the chemical formula: BaZrO₃, a compositeoxide represented by the chemical formula: Ba(Ce,Zr)O₃, and a compositeoxide represented by the chemical formula: CaZrO₃.

The solid solution-forming ratio of platinum was determined on thecomposite oxide catalyst B by the same method as described for thecomposite oxide A. As a result, the solid solution-forming ratio ofplatinum in the composite oxide catalyst B was 38%.

<Preparation of Exhaust Gas-Purifying Catalyst BA>

An exhaust gas-purifying catalyst in the form of pellets was prepared bythe same method as described for the exhaust gas-purifying catalyst AAexcept that the composite oxide catalyst B was used instead of thecomposite oxide catalyst A. Hereinafter, the exhaust gas-purifyingcatalyst is referred to as an exhaust gas-purifying catalyst BA.

EXAMPLE 3 Preparation of Composite Oxide Catalyst C

Calcium acetate was dissolved into 100 mL of deionized water. Acomposite oxide catalyst powder was prepared by the same method asdescribed for the composite oxide catalyst A except that this aqueoussolution was used instead of the aqueous solution of barium acetate.Note that concentration of calcium in the aqueous solution was adjustedsuch that the atomic ratio of calcium with respect to the sum of ceriumand zirconium was 20.0 atomic %. Hereinafter, the powder thus obtainedis referred to as a composite oxide catalyst C.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the composite oxide catalyst C. As aresult, it was proved that the composite oxide catalyst C contained acomposite oxide represented by the chemical formula: CaZrO₃.

The solid solution-forming ratio of platinum was determined on thecomposite oxide catalyst C by the same method as described for thecomposite oxide A. As a result, the solid solution-forming ratio ofplatinum in the composite oxide catalyst C was 25%.

<Preparation of Exhaust Gas-Purifying Catalyst CA>

An exhaust gas-purifying catalyst in the form of pellets was prepared bythe same method as described for the exhaust gas-purifying catalyst AAexcept that the composite oxide catalyst C was used instead of thecomposite oxide catalyst A. Hereinafter, the exhaust gas-purifyingcatalyst is referred to as an exhaust gas-purifying catalyst CA.

EXAMPLE 4 Preparation of Refractory Support B

Lanthanum nitrate [La(NO₃)₃.6H₂O] and aluminum nitrate [Al(NO₃)₃.9H₂O]were weighed such that the atomic ratio of lanthanum to aluminum was 1:1and were added to deionized water. Here, 145 g of lanthanum nitrate and125 g of aluminum nitrate were added to 1,500 mL of deionized water.

After stirring sufficiently, an aqueous solution containing ammoniumhydroxide was dropped into the aqueous solution at room temperature tocause coprecipitation. The aqueous solution containing the coprecipitatewas stirred sufficiently and then filtrated. Note that the aqueoussolution containing ammonium hydroxide was dropped such that 2 mol ofammonia was loaded.

The filter cake was sufficiently washed with deionized water and driedat 110° C. The dried material was subjected to a calcination at 600° C.for 3 hours in the atmosphere. The calcined material was crushed byusing a mortar and fired at 800° C. for 5 hours in the atmosphere.Hereinafter, the powder thus obtained is referred to as a refractorysupport B.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the refractory support B. As a result,it was proved that the refractory support B had a perovskite structurerepresented by a chemical formula: LaAlO₃. Note that the specificsurface area of the refractory support B was 60 m²/g.

<Preparation of Exhaust Gas-Purifying Catalyst AB>

An exhaust gas-purifying catalyst in the form of pellets was prepared bythe same method as described for the exhaust gas-purifying catalyst AAexcept that the refractory support B was used instead of the refractorysupport A. Hereinafter, the exhaust gas-purifying catalyst is referredto as an exhaust gas-purifying catalyst AB.

EXAMPLE 5 Preparation of Refractory Support C

Neodymium nitrate [Nd(NO₃)₃.6H₂O] and zirconium oxynitrate[ZrO(NO₃)₂.2H₂O] were weighed such that the atomic ratio of neodymium tozirconium was 1:1 and were added to deionized water. Here, 109.5 g ofneodymium nitrate and 66.8 g of zirconium oxynitrate were added to 1,000mL of deionized water.

After stirring sufficiently, an aqueous solution containing ammoniumhydroxide was dropped into the aqueous solution at room temperature tocause coprecipitation. The aqueous solution containing the coprecipitatewas stirred sufficiently and then filtrated. Note that the aqueoussolution containing ammonium hydroxide was dropped such that 3.5 mol ofammonia was loaded.

The filter cake was sufficiently washed with deionized water and driedat 110° C. The dried material was subjected to a calcination at 600° C.for 3 hours in the atmosphere. The calcined material was crushed byusing a mortar and fired at 900° C. for 5 hours in the atmosphere.Hereinafter, the powder thus obtained is referred to as a refractorysupport C.

The measurement of diffraction spectrum utilizing an X-raydiffractometer was carried out on the refractory support C. As a result,it was proved that the refractory support C had a pyrochlore structurerepresented by a chemical formula: Nd₂Zr₂O₇. Note that the specificsurface area of the refractory support C was 45 m²/g.

<Preparation of Exhaust Gas-Purifying Catalyst AC>

An exhaust gas-purifying catalyst in the form of pellets was prepared bythe same method as described for the exhaust gas-purifying catalyst AAexcept that the refractory support C was used instead of the refractorysupport A. Hereinafter, the exhaust gas-purifying catalyst is referredto as an exhaust gas-purifying catalyst AC.

COMPARATIVE EXAMPLE Preparation of Exhaust Gas-Purifying Catalyst AD

An exhaust gas-purifying catalyst in the form of pellets was prepared bythe same method as described for the exhaust gas-purifying catalyst AAexcept that a commercially available alumina with the specific surfacearea of 90 m²/g was used as a refractory support D instead of therefractory support A. Hereinafter, the exhaust gas-purifying catalyst isreferred to as an exhaust gas-purifying catalyst AD.

Next, the endurance of these exhaust gas-purifying catalysts was testedby the following method.

First, each exhaust gas-purifying catalyst was set in a flow-typeendurance test apparatus, and a gas containing nitrogen as a maincomponent was made to flow through the catalyst bed at a flow rate of1000 mL/minute for 30 hours. The temperature of the catalyst bed washeld at 1050° C. As the gas made to flow through the catalyst bed, alean gas prepared by adding oxygen to nitrogen at a concentration of 5%and a rich gas prepared by adding carbon monoxide to nitrogen at aconcentration of 10% were used, and these gases were switched atintervals of 5 minutes.

Next, each exhaust gas-purifying catalyst was set in an atmosphericfixed bed flow reactor. Then, the temperature of the catalyst bed wasraised from 100° to 500° C. at the temperature increase rate of 12°C./minute and the exhaust gas-purifying ratio was continuously measuredwhile a model gas was made to flow through the catalyst bed. As themodel gas, the gas containing equivalent amounts of oxidizing components(oxygen and nitrogen oxides) and reducing components (carbon monoxide,hydrocarbons and hydrogen), which were adjusted stoichiometrically, wasused. The results were shown in the table below.

TABLE 1 50% Exhaust Composition of composite purifying gas- oxidecatalyst Composition temprature purifying Ce Zr Ba Ca Pt of refractory(° C.) catalyst (at %) (at %) (at %) (at %) (wt %) support HC NO_(X) AA50 50 20 0 1 MgAl₂O4 310 326 BA 50 50 10 10 1 MgAl₂O4 315 330 CA 50 50 020 1 MgAl₂O4 325 345 AB 50 50 20 0 1 LaAlO₃ 316 330 AC 50 50 20 0 1Nd₂ZrO₇ 322 335 AD 50 50 20 0 1 Al₂O₃ 405 440

In the above table, the columns denoted by “Ba”, “Ce”, “Zr” and “Ca”show the atomic ratios of barium, cerium, zirconium and calcium withrespect to metal elements other than platinum contained in the compositeoxide catalyst, respectively. The column denoted by “Pt” shows theweight ratio of platinum with respect to the composite oxide catalyst.The column denoted by “50% purifying temperature” shows the lowesttemperature of the catalyst bed at which 50% or more of each componentcontained in the model gas was purified, and the columns denoted by “HC”and “NO_(x)” show the data for hydrocarbons and nitrogen oxides,respectively.

As shown in the table, the exhaust gas-purifying catalysts AA, BA, CA,AB and AC could purify the model gas at lower temperatures as comparedto the exhaust gas-purifying catalyst AD. This result revealed that theexhaust gas-purifying catalysts AA, BA, CA, AB and AC were excellent inendurance as compared to the exhaust gas-purifying catalyst AD.

Next, the measurements of diffraction spectrum utilizing the X-raydiffractometer were carried out on the exhaust gas-purifying catalystsAA and AD after the endurance test. The results are shown in FIG. 4.

FIG. 4 is a graph showing X-ray diffraction spectra of exhaustgas-purifying catalysts obtained after an endurance test. In the figure,the abscissa denotes the diffraction angle, while the ordinate denotesthe detected intensity. Also, in the figure, the curve S_(AA) representsthe X-ray diffraction spectrum obtained on the exhaust gas-purifyingcatalyst AA after the endurance test, and the curve SAD represents theX-ray diffraction spectrum obtained on the exhaust gas-purifyingcatalyst AD after the endurance test.

The spectrum S_(AA) does not includes the peak originated from thecomposite oxide represented by the chemical formula: BaAl₂O₄. Incontrast, the spectrum SAD includes the peak originated from thecomposite oxide represented by the chemical formula: BaAl₂O₄. Althoughnot shown in the figure, both spectra obtained on the exhaustgas-purifying catalysts AA and AD before the endurance test do notincludes the peak originated from the composite oxide represented by thechemical formula: BaAl₂O₄.

As apparent from this, the decomposition of the composite oxide catalystA occurred in the exhaust gas-purifying catalyst AD by the endurancetest so as to produce the composite oxide represented by the chemicalformula: BaAl₂O₄ as a decomposition product. In contrast, in the exhaustgas-purifying catalyst AA, no decomposition of the composite oxidecatalyst A was occurred by the endurance test.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An exhaust gas-purifying catalyst comprising: a composite oxidecatalyst containing a rare-earth element, an alkaline-earth element anda precious metal, a part of the rare-earth element and a part of thealkaline-earth element forming a composite oxide, and the compositeoxide and a part of the precious metal forming a solid solution; and arefractory support supporting the composite oxide catalyst and includingat least one composite oxide selected from the group consisting of afirst composite oxide represented by a general formula AB₂O₄, a secondcomposite oxide having a perovskite structure represented by a generalformula LMO₃, and a third composite oxide having a pyrochlore structurerepresented by a general formula X₂Y₂O₇, the element A beingalkaline-earth element and/or transition metal element, the element Bbeing at least one element selected from the group consisting ofaluminum, magnesium and transition metal elements and differing from theelement A, the element L being rare-earth element and/or alkaline-earthelement, the element M being aluminum and/or transition metal element,the element X being rare-earth element, and the element Y beingzirconium and/or titanium.
 2. An exhaust gas-purifying catalystcomprising: a composite oxide catalyst containing a rare-earth element,an alkaline-earth element, zirconium and a precious metal, a part of therare-earth element and a part of zirconium forming a composite oxidewith at least a part of the alkaline-earth element, and the compositeoxide and a part of the precious metal forming a solid solution; and arefractory support supporting the composite oxide catalyst and includingat least one composite oxide selected from the group consisting of afirst composite oxide represented by a general formula AB₂O₄, a secondcomposite oxide having a perovskite structure represented by a generalformula LMO₃, and a third composite oxide having a pyrochlore structurerepresented by a general formula X₂Y₂O₇, the element A beingalkaline-earth element and/or transition metal element, the element Bbeing at least one element selected from the group consisting ofaluminum, magnesium and transition metal elements and differing from theelement A, the element L being rare-earth element and/or alkaline-earthelement, the element M being aluminum and/or transition metal element,the element X being rare-earth element, and the element Y beingzirconium and/or titanium.
 3. The exhaust gas-purifying catalystaccording to claim 1 or 2, wherein the refractory support includes thefirst composite oxide, the first composite oxide has a spinel structure,and the element B includes aluminum.
 4. The exhaust gas-purifyingcatalyst according to claim 1 or 2, wherein the refractory supportincludes the second composite oxide, and the element M includesaluminum.
 5. The exhaust gas-purifying catalyst according to claim 1 or2, wherein the refractory support includes the third composite oxide,and the element Y includes zirconium.